POLYMORPHID^. 
285 
Bumortieria Moorei ; and, unless I have succeeded in proving the gradual steps by 
which these two species have been evolved, the reader will naturally fail to see 
why they are separated not only as different species, and different genera, but 
even as belonging to different families. Though there are slight — very slight — 
differences by which the individuals of tlie two species may be detected, yet the 
whole value of these differences may be said to depend on the genealogy. 
The table indicates that Dum. prisca probably splits up into four branches — 
Dim. 'pseudoradiosa, which is somewhat doubtful. Bum. Levesquei, Bum. costula, 
and Bum. sparsicosta which are fairly certain. Bum. sparsicosta is not derived 
from Bum. Levesquei, because the ribs in its inner whorls are decidedly too 
distant ; neither is it derived from Bum. costula, because it shows no traces of the 
peculiar " spaced " ribbing of the outer whorls of that species. Bum. Levesquei, 
again, splits up into five branches : — (1) culminating in "Moorei" to which every 
gradation can be traced through Bum. radians ; (2) exhibiting a series of 
changes through Bum. striatulocostata to the very Grammoceratan-like form Bum. 
striatulocosta |3, of which all the links are complete ; (3) producing Bum. radiosa, 
a kind of senile development of Bum. Levesqiiei, attaining fine striae and involute 
whorls ; (4) showing the peculiar evolute form Bum. subundulata ; (5) probably 
leading to the late-appearing form Bum. grammoceroides, of which the links are 
missing. Thus the species of Bumortieria are seen to be so many parallel genetic 
series carrying on independent development ; and the specimens which are figured 
in this Monograph may be considered a very fairly complete series illustrating 
specific evolution. 
That Tmetoceras is descended from Pol. Vernosse there can be little doubt ; and 
that it came through Pol. Zitteli is very probable, because, though our specimens 
do not show the character, the Continental examples have periodic constrictions 
like Pol. Zitteli. Tmetoceras scissum appears to have existed for rather a long 
time ; but in the higher zone appreciable changes in the flattening and involution 
of the whorls and the approximation of the ribs become decidedly apparent, 
giving rise to a form (PI. XLYIII, figs. 8 — 10) which is really a variety of the 
type, and this passes onwards into the more specialised form Tmetoceras Hollandse, 
probably the last species of the genus. There is, however, another branch from 
Tmetoc. scissum, namely the form with occasional bifurcate ribs to which Haug 
has applied the specific name Sutneri. 
That Gatulloceras came also through. Pol. Zitteli is extremely probable ; and 
it shows the same periodic constrictions. That later than Pol. Zitteli there was 
a species which was the common parent of Tmetoceras and Gatulloceras is also 
likely. This parent again may have split into two branches, one developing 
Gatidl. Bumortieri — from which come Gatull. Leesbergi, more involute, and Gatull. 
insignisimile, more coarsly-ribbed ; and the other into the furrowed Gatull. Perroudi 
